1. Field of the Invention
The present invention relates to electronic document imaging. More particularly, the present invention is directed to a linear electronic document imaging device as used in an image reading apparatus such as copying apparatus, facsimile apparatus, scanner and electronic blackboard.
2. Related Background Art
To facilitate explaining the background leading to the present invention, a typical traditional lightguide electronic document imager 10 is illustrated here in FIG. 1A and FIG. 2A. The traditional lightguide electronic document imager 10 has an imager frame 30 whose top part has a scan line backing portion 33 with a transparent window 34 located therein. The top surface of the scan line backing portion 33 and the transparent window 34 are both flat for supporting an advancing document 2 with its image side down and along a transport direction as indicated by an arrow. The mechanism for advancing the document 2 is well known in the art, including for example stepper motor and drive rollers, and not shown here. To facilitate description, a background X-Y-Z Cartesian coordinates and an r-θ polar coordinates are added with the r-axis coincides with the X-axis and the θ-coordinate incrementing clockwise. Thus, as the document 2 is advanced atop the transparent window 34, the image side of the document 2 gets scanned by the traditional lightguide electronic document imager 10 line-by-line with a scan line 4 lies parallel to the Z-axis and centered at X=Y=0. Below the scan line backing portion 33 and located inside the imager frame 30 are a lightguide 13, a rod lens 20 and a line image sensor module 11. Atop the line image sensor module 11 is a line image sensing area 12 and associated built-in signal conversion circuitry (not shown) for converting an incident line image light into a corresponding video signal output 14. The lightguide 13 typically includes internal light sources such as red light source 151R, blue light source 151B and green light source 151G. The lightguide 13 converts emissions from the internal light sources into line-illumination 17 generally aiming at the scan line 4. The line-illumination 17 then gets image-wise reflected by the image side of the document 2 along the scan line 4 and focused by the rod lens 20 into incident line image 5 ultimately focusing onto the line image sensing area 12 thus consequently converted into the video signal output 14. For convenience of technical description to be presently described, an imaging parameter called imaging distance 15 (IMD) is shown that is defined as the distance between the scan line 4 and the focal point of the line image sensing area 12. A highly important and well known performance parameter for an electronic document imager is its output signal-to-noise ratio (S/N) as measured from the video signal output 14. Accordingly, it is a primary object of the present invention to provide an improved lightguide electronic document imager with a higher S/N.